Glutamine as a γ-glutamyl donor for γ-glutamyl transpeptidase: γ-glutamyl peptide formation

Abstract

This study demonstrates the formation of γ-glutamyl peptides from glutamine and plasma amino acids, as catalyzed by γ-glutamyl transpeptidase. It also establishes the effect of various amino acids in modulating the rate of glutamine utilization as well as the hydrolytic or transfer

The mechanism of the utilization of glutamine as catalyzed by γ-glutamyl transpeptidase, involves the formation of a γ-glutamyl enzyme bound intermediate as the initial step, with release of the amide nitrogen as ammonium, NH₄⁺, Figure 1. The γ-glutamyl enzyme bound intermediate either reacts with the acceptor amino acids or water; reaction with amino acids yields γ-glutamylpeptides via the transfer pathway and reaction with water yields glutamate via the hydrolytic pathway.

Postulated reaction mechanism of the utilization of glutamine by γ-glutamyl transpeptidase and reaction products.

Renal γ-glutamyl transpeptidase. γGT is an extrinsic membrane bound glycoprotein, 95 percent being membraneous and most, but not all, associated with the lumial brush border membranes (1,2,3). γ-Glutamyl transpeptidase is also present at the antiluminal cell region (4,5,6). The significance of this assymetrical distribution is that the former catalyzes predominantly hydrolytic reaction, due to the limited delivery of the γ-glutamyl donor substrate while the latter catalyzes the transfer reaction due to the limited enzyme present (7).

Previous studies had demonstrated the utilization of glutamine by γ-glutamyl transpeptidase as both a donor and acceptor reactant (8). The focus of this study was the utilization of glutamine by γ-glutamyl transpeptidase in the presence of maleate, which shifts the reaction into a hydrolytic mode, consequently glutamate was the major end product measured, Figure 1. Physiologically, the enzyme is presented with γ-glutamyl donor substrates, glutamine or glutathione, in a mileau devoid of maleate. Therefore, I undertook these studies to determine the reactivity of the enzyme with glutamine and plasma amino acid in the absence of maleate. The results to follow, show that when glutamine alone was used as a substrate, it yielded predominantly the transfer product γ-glutamylglutamine. The effect of various amino acids on the utilization of glutamine was (1) to increase the flux of glutamine through the initial step, evidenced by the increase in ammonium, NH₄⁺, formation; (2) to influence the relative distribution of the γ-glutamyl moiety into the hydrolytic or transfer pathway. In addition, the formation of γ-glutamyl amino acids from glutamine is consistent with the physiological role of these complexes in transmembrane amino acid movements (9). product formed.